Conventionally, devices, such as radar apparatuses and fish finders, for transmitting and receiving a sound and/or an electromagnetic wave perform various kinds of processing to remove interference caused by another device(s). For example, JP2003-322678A discloses a method using the fish finder, in which interference is detected based on whether a ratio of an amplitude of a reception signal from a measurement (current measurement) and an amplitude of a reception signal from a previous measurement is below a predetermined value.
The interference signal is unlikely detected continuously for a plurality of times at the same level and the same depth, T he method disclosed in JP2003-322678A determines that the current reception signal is interference if the ratio of the amplitudes is above the predetermined value (i.e., if an intensity difference between the reception signals is large) and displays it on a display unit using the previous reception signal.
However, because the method disclosed in JP2003-322678A determines as interference as long as the intensity difference is large, there has been a possibility that a reception signal that is a normal echo other than interference (a target object such as school of fish) may be determined as interference. On the other hand, in another method with lower detection sensitivity than the method disclosed in JP2003-322678A, there has been a possibility that even the reception signal that originally is interference may not be determined as interference.